Due to a bug in our sidekiq middleware, post-update actions were not
being applied after changing assignees. This is fixed by changing
the format of the arguments to the background worker.

An unused worker is deleted, and we opt-in to the use of specialized
workers in the API, meaning we can delete some duplicated code.

Details of the fix:

This is vital to getting correct deduplication behavior in our sidekiq
middleware. The current logic is:

- CLIENT creates hash from worker queue name and arguments (calling
  `#to_s`)
- a job matching that hash is already running (in the working queue), it
  is not enqueued by the client.
- otherwise, the job is serialized and sent to the server
- SERVER pops the job and deserializes it
- the server runs the job
- The server calculates the deduplication hash, and deletes it, allowing
  similar jobs to be enqueued again.

This protocol relies on serialization being completely isomorphic. i.e.
`load . serialize === id`. If not, then the two hashes will not
match, and the deduplication block will remain up, and not be taken down
by the server.

The workaround in our case is to use a round-trippable hashmap (i.e.
string keys rather than symbols), since:

```
JSON.load({ 'x' => true}.to_json) == { 'x' => true }
```

but

```
JSON.load({ x: => true}.to_json) /= { x: => true }
```

See: https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/1090

Changelog: fixed